The present invention relates generally to x-ray imaging systems, and more particularly, to a system and method of reducing recovery time of solid-state x-ray detectors within an x-ray system.
An x-ray system typically includes an x-ray beam source and an x-ray detector. An x-ray beam is directed by the x-ray source across a region of interest of a patient and is detected by the x-ray detector, which in turn generates projection data in response to the x-ray beam.
A solid-state x-ray detector commonly has an array of pixels composed of field effect transistors (FETs) that perform as switches and photodiodes, which detect light. The FETs and the photodiodes are constructed of amorphous silicon, over which cesium iodide (CsI) is deposited. The CsI absorbs x-rays, generated by the x-ray beam source, and converts them into light energy, which is then detected by the photodiodes.
The photodiode due to its construction performs as a capacitor and stores energy in the form of charge. Initialization of the detector occurs prior to an x-ray exposure during a technique that is commonly referred to as “scrubbing the detector” or simply “scrubbing”. During scrubbing each photodiode is charged to a determined and initial voltage potential. The detector is then exposed to x-rays, which are absorbed by the CsI. Light that is emitted from the CsI is in proportion to flux of the x-rays and partially discharges the photodiodes. After conclusion of the exposure, voltage potential across the photodiodes is returned to the initial voltage potential. The amount of charge required to return the photodiodes to the initial voltage potential, is related to the x-ray dosage amount of a pixel integrated by a pixel coverage area for the length of an exposure.
The detector is read and scrubbed row by row, as controlled through active switching of the FETs. Reading is performed whenever an image produced by the detector contains valuable data, mainly images that contain exposure data or offset data. Since data acquired during scrubbing is not of interest it is discarded. Scrubbing is performed to maintain proper voltage bias across the photodiodes during idle periods or to perhaps reduce the effects of lag or incomplete charge restoration of the photodiodes.
There is a desire to minimize power dissipation within the x-ray system and the detector during the idle periods. One method of reducing power dissipation is to simply power off the detector between patients or between readings. However, since the detector is fabricated from amorphous silicon a substantial amount of time is required to stabilize the detector, causing a delay in use of the x-ray system.
Alternatively, part of the detector can be powered “OFF” to minimize stabilization time of the detector when full power is reapplied. In order to reduce this stabilization time, scan circuitry of the detector, which is used for scrubbing, is powered “ON” and is active. Readout circuitry of the detector, which is used to acquire data, can be powered OFF, because during idle time data is not acquired.
However, when powering OFF readout circuitry of an x-ray detector, voltage potential maintained across the photodiodes can change temporarily, potentially causing generation of a temporal error signal, which is added to valuable image or offset data until the maintained voltage potential stabilizes. This occurs since the cathode potential of the photodiodes is controlled by the readout circuitry via the data lines. When the readout circuitry is powered OFF, it no longer controls the potential of the data lines and therefore the potential of the cathodes of all the photodiodes, even when the potential of photodiode anodes remains constant. With the readout circuitry powered OFF, the data lines, and therefore the potentials of all the photodiode cathodes are free to drift toward the potential of the photodiode anodes, which is maintained during partial power OFF, and is the same potential for all of the photodiodes that form the detector.
When power is restored to the readout circuitry, the readout circuitry reestablishes an appropriate data line or photodiode cathode potential. Due to the parasitic impedance and capacitance of the data lines, the potential of the photodiode cathode requires a significant amount of time to stabilize. During this time, offset and image data is modified by the error signal.
Additionally, although data lines have associated readout channels that often maintain the data lines at a voltage potential level that is referred to as a virtual ground, the data lines can actually be slightly higher or lower than the virtual ground potential. This may be due to architecture, implementation, or simply process variation of a readout channel design.
It is therefore desirable to provide a method of reducing recovery time of an x-ray system detector so as to allow for reduced power dissipation. It is also desirable that the method minimizes generation and duration of error signals.